In a wireless communication system, both transmitter-side and receiver-side use multiple antennas to acquire a higher rate by way of spatial multiplexing. Compared with the common spatial multiplexing methods, an improved technology, in which the receiver-side feeds back the channel information to the transmitter-side and the transmitter-side uses transmitting precoding technology according to the acquired channel information, can largely improve the transmission performance. In a mode of Single-User Multi-input Multi-output (SU-MIMO), channel characteristic vector information is directly utilized to perform the precoding; in a mode of Multi-Users Multi-input Multi-output (MU-MIMO), the relatively accurate channel information is needed.
In 3GPP Long Term Evolution (or simply, LTE) plan, the feedback of channel information is mainly to utilize a relatively simple feedback method of a single codebook, and the performance of transmitting precoding technology of MIMO depends more on feedback accuracy of the codebook therein.
Herein, the fundamental of quantization feedback of channel information based on a codebook is set forth briefly as following:
Supposing the capacity of a limited feedback channel is B bps/Hz, the number of available code words is N=2B. The characteristic vector space of a channel matrix after quantization forms a codebook space ={F1, F2, . . . , FN}. A transmitter-side and a receiver-side jointly save or generate the codebook  (same in both transmitter-side and receiver-side) in real time. For each channel realization H, the receiver-side selects one code word {circumflex over (F)} that best matches the channel realization H from the codebook space  according to certain principles, and feeds back the sequence number i (code word sequence number) of the code word {circumflex over (F)} to the transmitter-side. Herein, the code word sequence number is called codebook Precoding Matrix Indicator (or simply, PMI). The transmitter-side seeks out the corresponding precoding code word {circumflex over (F)} according to this sequence number i, thereby acquiring corresponding channel information, and {circumflex over (F)} indicates characteristic vector information of the channel.
Normally, the codebook space  can be further divided into codebooks corresponding to a plurality of Ranks, and each Rank corresponds to a plurality of code words for quantizing the precoding matrix composed of characteristic vectors under the Rank. Since the Rank of a channel is equal to the number of non-zero characteristic vectors, the number of columns of code words normally is N when the Rank is N. Therefore, the code book space  can be divided into a plurality of sub-codebooks according to different Ranks, as shown in Table 1.
TABLE 1An illustration of a codebook being divided into a plurality of sub-codebooks according to Ranks The number of layer υ (Rank)12. . .N 1 2. . .  NA vector set ofA matrix set ofA matrix set of code words withcode words withcode words with the number ofthe number ofthe number ofcolumns being 1columns being 2columns being N
In this case, when Rank>1, all code words required to be stored are in a form of matrix;
wherein codebooks in a LTE protocol use this feedback method of codebook quantization, and a codebook of downlink 4 transmitting antennas of LTE is shown in Table 2; in fact, a precoding codebook in LTE and the codebook quantized by channel information have the same meaning. In the following, for unification, a vector can also be regarded as a matrix with one dimension.
TABLE 2An illustration of a codebook of downlink 4 transmitting antennas of LTECodebook The total number of layers υIndexun1234 0u0 = [1 −1 −1 −1]TW0{1} W0{14}/{square root over (2)}  W0{124}/{square root over (3)}  W0{1234}/2   1u1 = [1 −j 1 j]TW1{1} W1{12}/{square root over (2)}  W1{123}/{square root over (3)}  W1{1234}/2   2u2 = [1 1 −1 1]TW2{1} W2{12}/{square root over (2)}  W2{123}/{square root over (3)}  W2{3214}/2  3u3 = [1 j 1 −j]TW3{1} W3{12}/{square root over (2)}  W3{123}/{square root over (3)}  W3{3214}/2   4u4 = [1 (−1 − j)/{square root over (2)} −j (1 − j)/{square root over (2)}]TW4{1} W4{14}/{square root over (2)}  W4{124}/{square root over (3)}  W4{1234}/2   5u5 = [1 (1 − j)/{square root over (2)} j (−1 − j)/{square root over (2)}]TW5{1} W5{14}/{square root over (2)}  W5{124}/{square root over (3)}  W5{1234}/2   6u6 = [1 (1 + j)/{square root over (2)} −j (−1 + j)/{square root over (2)}]TW6{1} W6{13}/{square root over (2)}  W6{134}/{square root over (3)}  W6{1324}/2   7u7 = [1 (−1 + j)/{square root over (2)} j (1 + j)/{square root over (2)}]TW7{1} W7{13}/{square root over (2)}  W7{134}/{square root over (3)}  W7{1324}/2   8u8 = [1 −1 1 1]TW8{1} W8{12}/{square root over (2)}  W8{124}/{square root over (3)}  W8{1234}/2   9u9 = [1 −j −1 −j]TW9{1} W9{14}/{square root over (2)}  W9{134}/{square root over (3)}  W9{1234}/2  10u10 = [1 1 1 −1]TW10{1}W10{13}/{square root over (2)} W10{123}/{square root over (3)}W10{1324}/211u11 = [1 j −1 j]TW11{1}W11{13}/{square root over (2)} W11{134}/{square root over (3)}W11{1324}/212u12 = [1 −1 −1 1]TW12{1}W12{12}/{square root over (2)}W12{123}/{square root over (3)}W12{1234}/213u13 = [1 −1 1 −1]TW13{1}W13{13}/{square root over (2)}W13{123}/{square root over (3)}W13{1324}/214u14 = [1 1 −1 −1]TW14{1}W14{13}/{square root over (2)}W14{123}/{square root over (3)}W14{3214}/215u15 = [1 1 1 1]TW15{1}W15{12}/{square root over (2)}W15{123}/{square root over (3)}W15{1234}/2
In this case, for Wn=I−2ununH|unHun, I is a unit matrix, and Wk(j) represents jth column of vector of matrix Wk. Wk(j1, j2, . . . jn) represents a matrix composed of columns j1, j2, . . . , jn of matrix Wk, and unH represents a conjugate transpose matrix of un; wherein n denotes the sequence number with a value from 0 to 15.
With the development of communication technology, in advanced Long Term Evolution (LTE-Advanced), higher requirements have been put forward for spectrum efficiency, and as a result the number of antennas is also increased to 8 and a codebook feedback of 8 transmitting antennas is needed to be designed to perform the quantization feedback of channel information.
In the standards of LTE, a minimum feedback unit of channel information is a subband, and one subband is composed of several Resource Blocks (RB), and each RB is composed of a plurality of Resource Elements (RE). RE is a minimum unit of time-frequency resource in LTE, and a resource representation method of LTE is still used in LTE-A.
In the related art, there is an idea that is very different with previous feedback mode of a single codebook, and it can be described as: a user equipment (UE) feeds back one first Precoding Matrix Indicator (PMI1) information, and a base station seeks out a corresponding code word WPMI1 in codebook C1 according to PMI1; the UE also feeds back other one or more Precoding Matrix Indicator (PMI) information that are represented as PMI2 to PMIn (respectively called second Precoding Matrix Indicator information, . . . , nth Precoding Matrix Indicator information); the base station seeks out the corresponding code words WPMI2 to WPMIn from the corresponding codebooks, and constructs one matrix by using a function relationship F(WPMI1, WPMI2, . . . WPMIn) to indicate the channel information of a subband. Take 2 PMI for example, the channel information can be represented by a plurality of function relationships such as product or Kronecker product (its operator is represented by ) and the like:
      [                                        WPMI            ⁢                                                  ⁢            1                                    O                                      O                                      WPMI            ⁢                                                  ⁢            1                                ]    ⁢  WPMI  ⁢          ⁢  2or WPMI1×WPMI2, WPMI2WPMI1 and the similar function relationships.
Feedbacks of a plurality of PMI can be used to respectively configure feedback periods of various PMI, thereby utilizing the feedback resource effectively, and it can also improve feedback precision effectively, without needing to design one single codebook having a large number of code words, and this method reduces the complexity of design of codebook.
However, a problem existing in the related art is that, this feedback mode is required to be used in all cases, and in the case of higher Rank, the performance gain brought by feeding back PMI2 to characterize the changed WPMI2 is limited. Moreover, in the case of a large number of feedbacks of PMI2 and high Rank, more applications are in a mode of single-user MIMO (SU-MIMO), and their requirement for feedback precision is relatively low.